To ascertain whether gap junctions are preferentially formed by proliferated retinal stem cells, an analysis by electron microscopic autoradiography was made on the retina of the Xenopus embryo at stages 26, 29 and 31, after pulse labelling with 3H-thymidine. When the administration of the isotope was carried out for 0.5 or 3 h, retinal cells in S phase or S to M phase in the cell cycle were marked with the isotope, respectively. In these specimens, most gap junctions were found on the isotope-labelled cells, and few on the unlabelled cells. Several cells in mitosis also formed gap junctions. These results would suggest that gap junctions are formed by retinal stem cells in S to M phases of the cell cycle, but not by cells in G1 phase and post-mitotic cells.